First principles study of the electronic structure and Li-ion diffusion properties of co-doped LIFex-1MxPyNy-1O4 (M=Co/Mn, N--S/Si) Li-ion battery cathode materials

In this work, a first-principles method based on density functional theory was systematically employed to investigate the stability, electronic properties, lithium-ion migration rates, and capacity-voltage curves of the LiFe x-1 M x P y N y-1 O 4 (M = Co/Mn, N--S/Si) system. The results indicate that the lattice constants of the LiFe x-1 M x P y N y-1 O 4 (M = Co/Mn, N--S/Si) system show little variation, and the system exhibits low formation and binding energies. Among the investigated systems, LFP-Mn/S demonstrates the best structural and thermodynamic stability. The bandgap of the doped systems decreases, leading to enhanced electronic conductivity. The LiFe 0.875 Co 0.125 P 0.875 Si 0.125 O 4 and LiFe 0.875 Mn 0.125 P 0.875 Si 0.125 O 4 systems remain semiconductors, while the LiFe 0.875 Co 0.125 P 0.875 S 0.125 O 4 and LiFe 0.875 Mn 0.125 P 0.875 S 0.125 O 4 systems exhibit semi-metallic properties due to the introduction of sulfur. Differential charge density calculations reveal changes in the covalent bond strength of the doped systems, with the introduction of Si and S respectively increasing and decreasing the covalency of their bonds with surrounding oxygen atoms. Additionally, doping reduces the Li-ion diffusion energy barriers, with the LiFe 0.875 Co 0.125 P 0.875 Si 0.125 O 4 system exhibiting the lowest migration energy barrier. The Li- ion diffusion rate is four orders of magnitude faster than that of the intrinsic system. This is attributed to changes in the average lengths of Li-O, Co-O, and Fe-O bonds. Finally, doping also alters the de-lithiation voltage, with values ranging from 2.69 V to 3.65 V for the doped systems, and the LiFe 0.875 Co 0.125 P 0.875 Si 0.125 O 4 system shows the highest complete de-lithiation voltage of 3.65 V. The overall performance improvements of the doped system have significant implications for enhancing the performance of Li-ion batteries.